Electron beam amplifying tube with low noise electron gun



P. A. H. HART Aug. 23, 1966 ELECTRON BEAM AMPLIFYING TUBE WITH LOW NOISE ELECTRON GUN 2 Sheets-Sheet 1 m N T E 7 N T G 3 ER A vA A. V///// L my PB Filed Oct. 2, 1962 Aug. 3. 196 P. A. H. HART 3,268,754

ELECTRON BEAM AMPLIFYING TUBE WITH LOW NOISE ELECTRON GUN Filed Oct. ,2, 1962 2 Sheets-Sheet 2 INVENTOR PAUL A.H. HART M AGENT United States Patent Office Patented August 23, 1966 3 26 8,7 54 ELECTRON BEAM AMPLIFYINQ TUBE WITH LOW NOISE ELECTRON GUN Paul Anton Herman Hart, Emmasingel, Eindhoven, Netherlands, assignor to North American Philips Company, Inc., New York, N.Y., a corporation of Delaware Filed Oct. 2, 1962, Ser. No. 227,868 Claims priority, application Netherlands, Oct. 4, 1961, 269,926 4 Claims. (Cl. 31384) This invention relates to a device comprising an electron-beam tube for signal amplification by means of transverse electric interaction with an electron beam emanating from an electron gun, which beam is focussed by an axial magnetic field. In an electron-beam tube for such a device, for example, three coupling members are arranged between the electron gun and a collector electrode in the direction of movement of the electrons, the signal to amplified being fed to the first coupling membet, a pump signal supplying the energy for the amplification being fed to the second coupling member and the amplified signal being derived from the third coupling member. In order that the ampified signal derived from the third coupling member originates from the signal to be amplified, which signal is fed to the first coupling member, it is important that the electron beam at the first coupling member contains substantially no noise of signal frequency.

In the first coupling member the electrons, under the influence of the axial magnetic field and the transverse electric field produced by the signal to be amplified, perform a helical movement with a circulating frequency which is termed the cyclotron frequency f and which is a function of the axial magnetic field. If the cyclotron frequency f is not equal to the signal frequency f,, it appears that the signal propagation can be described by means of two transverse waves which are termed the fast and the slow wave respectively. It is possible to choose the shape and the proportions of the first coupling member so that at least theoretically only one of the two waves is produced. The same holds for the propagation of the noise along the beam: the propagation of the noise with a frequency differing from the cyclotron frequency may also be described by means of two transverse waves. It, therefore, is sufficient for the electron beam to contain substantially no noise of signal frequency at the first coupling device in the form in which the signal propagation takes place.

If the signal propagation takes place only in the form of a fast wave, the rapid wave components of the noise with signal frequency must consequently be removed from the electron beam. This may be effected, for example, by a suitable choice of the load impedance of the first coupling device. It is also possible to include an additional coupling member between the electron gun and the first coupling member which additional member serves for absorbing the fast Wave components of the noise at the signal frequency of the beam. In this case it does not matter how large the noise at the signal frequency on the electron beam emanating from the electron gun is. If the device serves for amplifying signals in a frequency band, the noise of the beam must naturally be suppressed for all frequencies in this band. However, this cannot be effected to the same extent for all the frequencies in the manner described: in the centre of the frequency band used the noise suppression can be achieved, but on the edge of the frequency range covered by a given tube, the noise suppression cannot be complete. For this reason and to prevent that a careful adjustment in the centre of the band is necessary, it is desirable that the noise in the relative frequency range in the electron beam emanating from the electron gun is as small as possible.

If the signal propagation takes place only in the form of a slow wave, the slow wave components of the noise at the signal frequency would consequently have to be removed from the electron beam. However, this is not possible in the manner described in the preceding paragraph. In this case also it is desirable that the noise in the relative frequency range on the electron beam emanating from the electron gun be as small as possible.

The device according to the invention provides an electron beam emanating from the electron gun having a very low noise in the relative frequency range and this device is based on the following recognition. In the propagation of signals and consequently also of noise in the beam, two kinds of transverse velocities must be reckoned with: first, that of the movement of the centre of gravity of the beam around the axis and, secondly the velocities of the electrons around the centre of gravity of the beam. The movement of the centre of gravity of the beam corresponds to the signal or the noise. The movement of the electrons around the centre of gravity of the beam is related indirectly only to the noise. A temperature may be assigned to the fluctuations of these movements. When leaving the cathode, the temperatures of the fluctuations of these movements are equal to the effective cathode temperature. It is possible to produce an interaction of energy between the fluctuations of the movement of the centre of gravity and the internal movement of electrons for all frequencies, the cyclotron frequency excepted, if a comparatively large spreading of axial velocity components is present in the beam. Then the same temperature of both movements results. If the temperature of the fluctuation of the internal movement of electrons between the cathode and the first coupling device is smaller than the temperature of the fluctuation of the movement of the centre of gravity, the interaction of energy is of advantage because just the movement of the centre of gravity corresponds to the noise. The difference in the fluctuations of the two movements may be formed in various manners. On the one hand it is possible that the fluctuations of the movement of the centre of gravity are increased, for example in the case of Brillouin focussing, as a result of the step in the magnetic field. For, in this case, the magnetic field has a very low value as a result of a particular screening of the cathode and it increases by steps until the value which prevails between the signal coupling devices. On the other hand it is possible that the fluctuations of the internal movement of electrons are reduced as a result of beam expansion in transverse direction. Since the signal frequency often does not differ much from the cyclotron frequency, it can just not be effected in the greater part of the tube, as a result of a comparatively large spreading of axial velocity components, that the interaction of energy for the frequency range in question takes place. It has appeared to be of importance to produce a comparatively large spreading of axial velocity components in a range in which the value of the magnetic field differs from that of the field between the signal coupling members. The comparatively large spreading of axial velocity components can be obtained by giving the beam at low axial velocity.

According to the invention such means are present that the electron beam expands as a result of the retardation of the beam in a range of the electron gun in which the value of the magnetic field differs from that of the field between the signal coupling members. Then, if required, the beam may be converged again in a magnetic field which is so strong that the associated cyclotron frequency is the same as that in the further system of the tube. This is an advantage over other methods to decrease the noise on the beam, for example a diaphragm, because the total beam current remains equal. The noise suppression of the beam obtained in the device according to the invention takes place both with respect to the fast and the slow wave, so that it is not necessary to take different measures for these cases.

A retardation of the electron beam in a range of the electron gun in which the value of the magnetic field differs from that of the field between the signal coupling members may be effected in various manners. If focussing according to Brillouin takes place in the electron gun, such a magnetic field is already present in the gun, namely at the variation by steps of the magnetic field strength from a very low value to the value of the field between the signal coupling devices. The voltages of the anodes of the electron gun in this case are such that the electron beam expands as a result of the retardation of the beam in a range in which the value of the magnetic field differs from that of the field between the signal coupling members.

If no focussing according to Brillouin takes place, the deviating value of the magnetic field may be effected if a ring of ferromagnetic material is provided around part of the cathode of the electron gun. By varying the place of the ring with respect to the cathode surface, a favourable compromise may be chosen between the high space charge density in the beam which requires a magnetic field through the cathode approaching zero, as is the case with Brillouin focussing, and a low initial noise temperature which on the contrary requires a high magnetic field through the cathode. In this case also, the voltages of the anodes should be chosen so that expansion occurs at the change of the value of the magnetic field.

The invention further relates to an electron beam tube for use in such a device. The invention relates in particular to an electron beam tube in which a ring of ferromagnetic material is provided around part of the cathode of the electron gun. In addition the invention relates to an electron gun for such an electron beam tube in which a ring of ferromagnetic material is provided around part of the cathode.

In order that the invention may readily be carried into effect, one embodiment thereof will now be described more fully, by way of example, with reference to the accompanying drawings, in which FIGURE 1 is a diagrammatic representation of an electron beam tube for signal amplification;

FIGURE 2 is a cross-section of an electron gun for use in a device according to the invention;

FIGURE 3 shows part of FIGURE 2 on an enlarged scale;

FIGURE 4 is a cross-section of an electron gun according to the invention.

FIGURE 1 is a diagrammatic representation of an electron beam tube for signal amplification for use in a device according to the invention. The evacuated envelope (not shown) comprises an electron gun 4 for producing and a collector 5 for collecting the electron beam. Both the envelope and the collector electrode may be of known structure.

An axial constant magnetic field, indicated by the arrow 6, is produced in known manner by means of an electromaguet or permanent magnet (not shown) provided outside the tube.

In the direction of the electron gun 4 towards the collector electrode 5 are arranged along the beam successively the first coupling member 1, the second coupling member 2 and the third coupling member 3.

The coupling member 1 comprises two parallel metal plates 7 and 8 connected together by a coil 9 and arranged on either side of and in parallel with the beam.

In parallel with at least a part of the coil 9 the signal source (not shown) is connected through two conductors 10, which source supplies the signal to be amplified having the frequency f The coupling member 3 is of a corresponding type and consists of two metal plates 11 and 12 connected through the coil 13. The amplified signal having the frequency f is derived from at least part of the coil 13 through the conductors 15.

The second coupling member 2 comprises four metal plates 16, 17, 18 and 19 extending in parallel with the beam and having a convex curvature with respect to the beam in planes at right angles to the beam. The oppositely located plates 16 and 18 and also the plates 17 and 19 are connected together electrically. Between the pairs of plates an alternating voltage having the frequency 2 is supplied through the conductors 21, which voltage is produced by a generator (not shown).

The coupling members 1, 2 and 3 described are of the conventional type but may also be constructed differently. For example, the members 1 and 3 may consist, as is known, of resonant cavities, in which on the front and back side parts of the wall are bent inwards in the manner of a tag, in which the tag-shaped parts extend in parallel with each other so that between them a transverse field may be produced.

To avoid couplings between the various coupling members, diaphragms may be provided between these devices as indicated at 22 and 23.

The voltages at the electron gun 4 are chosen in a particular manner according to the invention and, if required, the electron gun may in addition be constructed in a particular manner.

FIGURE 2 is a cross-section of an electron gun to be used in a device according to the invention. It comprises a cathode 31 and anodes 32, 33, 34, 35, 36 and 37. The anode 32 consists of iron. It has a very small aperture and extends cylindrically in the direction of the cathode in a manner such that the latter is substanatially screened magnetically from the external magnetic field which serves for focussing the beam in the cathode-ray tube. The anodes 33 to 37 are mounted in a ceramic tube 38 and kept spaced by the ceramic rings 39. The ceramic tube 38 and the anode 32 are mounted in a constantan sleeve 40. The cathode support 41 of rustless steel is mounted between two ceramic rings 42 which support is kept spaced from the anode 32 by the spacing member 43 consisting of molybdenum.

FIGURE 3 diagrammatically shows part of FIGURE 2 on an enlarged scale. In this figure, the cathode is indicated by 31 and the anodes by 32, 33, 34, 35, 36 and 37. In this figure also the value of the magnetic field prevailing is plotted. In the broken-line parts, the value could not be established accurately. On the ordinate, the axis of the gun corresponds to the value of the magnetic field between the signal coupling members. The graph shows that the value of the magnetic field varies within this gun focussed according to Brillouin. According to the invention, the voltages at the anodes 32 to 37 are such that the beam is retarded there and consequently expands. This may be realized by means of the adjustments stated in the table below. The magnetic field is so strong that the cyclotron frequency corresponding to it between the signal coupling members is 550 mc./s. In the table, also the noise temperature of the beam emerging from the gun is stated at 550 mc/s. If the particular adjustment according to the invention is not used, the noise temperature of the beam emanating from the gun at 550 mc./s. is at least 8400 K.

FIGURE 4 is a cross-section of an electron gun accord ing to the invention. It comprises a cathode 51 and anodes 52, 53, 54, 55, S6 and 57 consisting of non-magnetic material, for example molybdenum. The anode 52 has a very small aperture and extends cylindrically in the direction of the cathode. The anodes 53 to 57 are mounted in a ceramic tube 58 and kept spaced by the ceramic rings 59. The ceramic tube 58 and the anode 52 are mounted in a constantan sleeve 60. The cathode support '61 of rustless steel is mounted between two ceramic rings 62 and 63 which is kept spaced from the anode 52 by the ceramic spacing member 64. A ring 65 of magnetic material, for example, iron, is mounted within the annular spacing member 64 which ring is kept spaced from the ceramic ring 62 by means of the ceramic tube 66. The ring 65 of magnetic material which is provided around part of the cathode causes the presence of a strongly inhomogeneous magnetic field in the gun.

What is claimed is:

1. An electron gun, having a cathode and a plurality of anodes, for use with an electron beam amplification tube which provides signal amplification by means of a transverse electrical interaction with an electron beam comprising means for establishing a uniform axial magnetic field in said tube for focusing said beam, said cathode supplying electrons, means for establishing in the gun vicinity a magnetic field having a magnitude exceeding the said axial focusing field, and, means for energizing said anodes for expanding the beam by retardation in the gun vicinity where said magnetic field exceeds the magnitude of the focusing field to substantially reduce the noise component of the beam in the entire signal pass band.

2. An electron gun as set forth in claim 1 in which the means for establishing the field in the gun vicinity includes a ring of ferromagnetic material around part of the cathode for introducing an inhomogeneity in the magnetic field in the vicinity of the anodes.

3. An electron gun, having a cathode and a plurality of anodes, for use with an electron beam amplification tube which provides signal amplification by means of a transverse electrical interaction with an electron beam comprising means for establishing a uniform axial mag netic field in said tube for focusing said beam and providing said beam with a cyclotron frequency (fc falling within the frequency pass band of the amplifying tube, said cathode supplying electrons, means for establishing in the gun vicinity a magnetic field providing the electron beam in that vicinity with a cyclotron frequency (fe falling outside the pass band of the amplifying tube, and, means for energizing said anodes for expanding the beam by retardation in that portion of the field providing the cyclotron frequency (fe to substantially reduce the noise component of the beam throughout the signal pass band of the tube.

4. An electron gun as set forth in claim 3 in which the means for establishing the field in the gun vicinity includes a ring of ferromagnetic material around part of the cathode for introducing an inhomogeneity in the magnetic field in the vicinity of the anodes.

References Cited by the Examiner UNITED STATES PATENTS Re. 24,794 3/1960 Quate 3l384 X 2,801,361 7/1957 Pierce 31384 X 2,909,704 10/1959 Peter 31382.l X 2,956,198 10/1960 Elder et a1. 31384 X 2,959,740 11/1960 Adler 3304.7 3,092,745 6/1963 Veith et al. 3153.5 X 3,178,646 4/1965 Ashkin 330-4.7

FOREIGN PATENTS 905,904 9/ 1962 Great Britain.

JAMES W. LAWRENCE, Primary Examiner.

ARTHUR GAUSS, DAVID J. GALVIN, GEORGE N.

WESTBY, Examiners.

C. O. GARDNER, R. SEGAL, Assistant Examiners. 

1. AN ELECTRON GUN, HAVING A CATHODE AND A PLURALITY OF ANODES, FOR USE WITH AN ELECTRON BEAM AMPLIFICATION TUBE WHICH PROVIDES SIGNAL AMPLIFICATION BY MEANS OF A TRANSVERSE ELECTRICAL INTERACTION WITH AN ELECTRON BEAM COMPRISING MEANS FOR ESTABLISHING A UNIFORM AXIAL MAGNETIC FIELD IN SAID TUBE FOR FOCUSING SAID BEAM, SAID CATHODE SUPPLYING ELECTRONS, MEANS FOR ESTABLISHING IN THE GUN VICINITY A MAGNETIC FIELD HAVING A MAGNITUDE EXCEEDING THE SAID AXIAL FOCUSING FIELD, AND, MEANS FOR ENERGIZING SAID ANODES FOR EXPANDING THE BEAM BY RETARDATION IN THE GUN VICINITY WHERE SAID MAGNETIC FIELD EXCEEDS THE MAGNITUDE OF THE FOCUSING FIELD TO SUBSTANTIALLY REDUCE THE NOISE COMPONENT OF THE BEAM IN THE ENTIRE SIGNAL PASS BAND. 